The Future of Lr34 in Modern, High-Input Wheat Breeding Programs

Lr34 is an important adult plant resistance gene that is effective against a number of wheat (Triticum aestivum L.) diseases and has proved durable for over 100 yr. However, the energy requirements of Lr34 resistance, combined with the pleiotrophic effect of leaf tip necrosis, can result in reduced yield potential. To evaluate the absence of Lr34 from elite breeding material, a cross was made between similar-yielding, milling wheat cultivars 'Conquest' (Lr34sus) and 'Bakker Gold' (Lr34res). Independent lines were carried forward and, at the F-4 stage, sister pairs were selected for the different homozygous alleles of the marker csLV34, corresponding to Lr34res or Lr34sus. These lines were further selfed to produce nine stable F-8 pairs (Lr34res and Lr34sus) for field trialing and evaluation over 3 yr under a range of fungicide treatments. In the absence of fungicide and with natural infections of stripe rust (Puccinia striiformis f.sp. tritici) and leaf rust (Puccinia triticina), Lr34res lines had lower disease severity, increased total yield, and better grain quality than Lr34sus lines. This pattern was reversed under full fungicide control, as Lr34sus lines displayed higher mean plot yield (2010: + 4.3%, p < 0.001; 2012: + 4.4%, p < 0.001) and possessed superior grain quality data than the Lr34res lines. The high-yield, high-input farming environment and the use of full fungicide control in New Zealand's current wheat cultivar trialing system means that genetic sources of rust resistance such as Lr34 provide little or no competitive advantage.